Synthesis and structures of 1,3,1′,3′-tetrabenzyl-2,2′-biimidazolidinylidenes (electron-rich alkenes), their aminal intermediates and their degradation products

Abstract

Benzyl (R) substituted enetetramines 9 and 3 have been studied. From HNR(CH₂)₂NRH and CH(NMe₂)₂OBu^t or CH(OMe)₂NMe₂, two new intermediates along the pathway to 9, namely the orthoamide 11 and the bis(orthoamide) 12 were isolated. Each of 11 and 12 was converted into 9 by refluxing in toluene. Photolysis of 9 yielded the isomer 10, while thermolysis of 9 gave the di(debenzylated) product 1,1′-dibenzyl-2,2′-biimidazoline 13. A route to 3 (R = R′ = CH₂Ph) similar to those used for 9, involving the condensation of 1,2-C₆H₄[N(R)H]₂ with CH(OMe)₂NMe₂, or the reaction between 1,3-dibenzylbenzimidazolium chloride 8 (X = Cl) and NaH, did not give the expected enetetramine 3 (the dibenzo-analogue of 9), the bis(debenzylated) product 15 being obtained instead. Heating the orthoamide 1,2-RNC₆H₄N(R)C(H)NMe₂, prepared from CH(NMe₂)₂OBu^t and 1,2-C₆H₄[N(H)R]₂, also gave 15. The reactions of S₈, PhNCS or KOH with a mixture of 8 (X = Cl) and NaH gave 17, 18 or 19, respectively, consistent with the transient formation in each reaction of the tetrabenzylenetetramine 3 (R = R′ = CH₂Ph). The molecular structure of each of the crystalline compounds 10, 11, 12 and 13 was established by X-ray diffraction.